Grease is used for the sliding portions in the structure parts of automobile components, OA/AV equipment and the like. Oil may separate from the grease composition, especially under the conditions of high temperatures. The structure parts may be stained with oil thus separating from the grease, which may cause some problems. In light of this, the grease used for the sliding portions in the structure parts of the automobile components, OA/AV equipment and the like is required to prevent the oil separation. The phenomenon of oil separation has a variety of modes, for example, oozing of oil from the grease, and collection of oil on the surface of grease (the so-called syneresis), which is observed after the grease is stored in a pail or drum for a long period of time.
It has been reported that addition of a styrene-based block copolymer can prevent the oil separation (JP 2002-327188 A, JP 2004-339447 A and JP 2007-297422 A).